This invention relates generally to injection molding, and particularly to a method of removing extraneous matter (e.g., a nonvolatile component of plastic substance) generated in a cavity of an injection mold comprised of a movable mold and an immovable mold, and a method of precisely and securely placing an insert in the injection mold.
In general, an injection-molding machine includes an injection mold comprised of a movable mold and an immovable mold, and moldably fluidized resins are injected into a mold cavity of the injection mold formed of the movable mold and the immovable mold, to form a casting. Between joint surfaces of the movable and immovable molds of a conventional injection mold is provided a small clearance through which air and gas in the cavity may be evacuated, and a gas vent is connected to the clearance. The clearance is configured to have such a small size that only air in the cavity and gas emitted from the fluidized resins (hereinafter referred to as xe2x80x9catmosphere in the cavityxe2x80x9d) may pass through the clearance, thus distributing the fluidized resins throughout whole space in the cavity.
To attach a part to a casting, the part may be joined integrally by means of thermal caulking with the casting that has been formed through an injection-molding process, or the part may be inserted in a mold during the injection-molding process to get integrally molded. However, the method of joining the part by means of thermal caulking has rarely been used because of low productivity thereof.
When a metal insert made of iron or containing great amounts of iron is embedded in the casting through the process of inserting the part in the mold, a concave holding portion in which the insert is placed is provided in the mold cavity. The holding portion may be magnetized for the purpose of securely holding the insert in the mold cavity. Alternatively, a magnet may be embedded in the holding portion to bold the insert.
FIGS. 12A and 12B illustrate a conventional handling device that places the insert in the holding portion. As shown in FIGS. 12A and 12B, the handling device 1 includes an attraction gripper 3 that attracts an insert 2, and an arm that moves and places the insert 2 attracted to the attraction gripper 3 at an insert position in an injection mold (not shown). The handling device 1 moves and rotates the arm 4 with the insert 2 attracted thereto in frontward, rearward, left-hand, right-hand, upward, and downward directions, and thereby properly positions the insert 2 in the injection mold (not shown).
The arm 4 is constituted, for example, of a jointed-arm robot; the attraction gripper 3, which is made of rubber in its entirety, is attached to a head 5 at a distal end of the arm 4. At a bottom of the attraction gripper 3 is provided an annular groove 3a between concentric inner and outer annular sections 3b and 3c to exert negative pressure on the insert 2. At a bottom of the annular groove 3a are provided a plurality of suction inlets 3d, 3d, . . . Each suction inlet 3d is connected to a negative pressure passage 3e in the head 5, and the negative pressure passage 3e is connected via a control valve (not shown) to a vacuum pump or vacuum tank (not shown).
When the handling device 1 is employed to position an insert 2 in the injection mold, first, the arm 4 is actuated to move and rotate in frontward, rearward, left-hand, right-hand, upward, and downward directions, to move the head 5 to a position where the insert 2 is picked up, so that the outer annular section 3b and the inner annular section 3c face target spots on the insert 2. This position being kept, the control valve is then switched to a position at which the negative pressure passage 3e opens connections to the vacuum pump or vacuum tank so that negative pressure is created in the annular groove 3a to exert an attraction.
The negative pressure attracts the insert 2 to the annular groove 3a. When the insert 2 is attracted to the annular groove 3a, the arm 4 is next operated to move and rotate in frontward, rearward, left-hand, right-hand, upward, and downward directions to move and attach the insert 2 to the holding portion in the injection mold. Subsequently, the control valve is switched to a position at which the negative pressure passage 3e opens connections to an atmosphere discharge port of the control valve to release the insert 2 from the attraction gripper 3. Thereafter, the arm 4 is operated to move the head 5 back to the position where the insert 2 is picked up.
Accordingly, a series of operations from the step of picking up the insert 2 to the step of moving the head 3 back to a home position is repeatedly performed for one cycle of the injection-molding process, with the result that productivity in embedding an insert in the casting may be enhanced in comparison with that which may be achieved through a thermal caulking process. To control the position of the arm 4 and the positioning of the insert 2, a contact sensor such as a microswitch, a relay, etc., or a noncontact sensor such as a magnetic sensor, an optical sensor, etc. may be used.
As described above, when the conventional injection mold is employed, fluidized resins are distributed throughout whole space in the cavity, and are solidified under such a condition as to allow entire inner surfaces of the cavity to be kept in full contact with the fluidized resins, so that castings without defect in outer surfaces or inner structures may be formed. However, nonvolatile components (e.g., flame retardant for suppressing propagation of a flame, additives for improving fluidity of resins, etc.) that exude from the fluidized resins may cool off and deposit on inner surfaces of the cavity, a land, and the like. The extraneous matters that deposit in the cavity may inhibit an atmosphere in the cavity from coming out, thus decreasing yields of the castings. In addition, the increased extraneous matters derived from nonvolatile components would disadvantageously adhere to the casting.
Accordingly, the extraneous matters derived from the nonvolatile components are removed once a day, or an evacuator circuit that evacuates the nonvolatile components outside by exerting negative pressure in the cavity is provided, so that the casting may be taken out of the cavity after the atmosphere in the cavity filled with fluidized resins is evacuated outside.
However, the former approach disadvantageously requires a temporal suspension of a line for a cleaning operation, and needs enormous manpower and time for dismantling the injection mold. On the other hand, the latter approach using an evacuator may fail to bring about sufficient cleaning effects by a scant one atmospheric pressure, thus decreasing reliability.
The use of the handling device 1 to locate the insert 2 at a holding portion in an injection mold where the insert 2 is held by a magnetic attraction of the holding portion, as described above, would make it possible to automate an insert positioning operation. However, if the insert 2 formed by performing a press-forming or stamping process assumes a curved or uneven shape as shown in FIG. 13A, the attraction gripper 3 may get into contact with a wrong spot on the insert 2 deviated from an appropriate spot to attract the insert 2, or a gap may be generated between the outer annular section 3b or the inner annular section 3c and a surface of the insert 2. Such a deviated spot of contact would require a delicate operation of correcting a position of the insert 2 by actuating the arm 4 to move and suspend in a finely modulated manner. Further, thus-generated gap would cause a negative pressure to decrease, and allow the insert 2 to fall off from the attraction gripper 3, disadvantageously resulting in failure to place the insert 2 in the holding portion. Otherwise, wear-out generated in the outer annular section 3b and the inner annular section 3c due to normal wear and tear or deterioration over time as shown in FIG. 13B would pose the same problem as described above.
Moreover, the insert 2 is likely to fall off from the holding portion due to oscillations or the like. In case where an inner surface of the holding portion is magnetized or a magnet is embedded in the holding portion to hold the insert using a magnetic force, the magnetic force is disadvantageously abated, thus making a magnetic attraction for holding the insert less than that which is exerted in case where a bare magnet is brought into direct contact with the insert to attract and hold the insert. More specifically, this is because a magnetic attraction of the magnet embedded in a surface of a mold cavity would abate because of a leakage of a magnetic flux into a mold made of metal materials.
The present invention has been made in order to eliminate the above disadvantages.
Therefore, it is an exemplified general object of the present invention to provide a method for improving yields of final castings and enhancing productivity in the injection-molding process.
Another exemplified and more specific object of the present invention to provide a method of discharging a nonvolatile component from a cavity of an injection mold without suspension of operations in an injection-molding line and without disassembling the injection mold.
Yet another exemplified object of the present invention is to provide a method of moving and placing an insert at a predetermined position in the injection mold without letting the insert fall off.
Yet another exemplified object of the present invention is to provide a method for preventing a magnetic attraction of a holding portion in the injection mold from abating due to a leakage of a magnetic flux.
In order to achieve the above objects, there is provided, as one aspect of the present invention, a method of removing extraneous matter in an injection mold having a cavity comprised of a movable mold and an immovable mold into which injection mold a resin is injected to form a casting. The method includes supplying a high-pressure gas into the cavity during a time period after completion of forming the casting till the injection mold opens partway, and allowing the high-pressure gas to jet out through a clearance formed between joint surfaces of the movable and immovable molds while the injection mold is opening, whereby a nonvolatile component of the resin is discharged.
According to this method, a high-pressure gas is supplied into the cavity after completion of forming the casting till the injection mold opens partway, so that the high-pressure gas is allowed to jet out through the clearance formed between joint surfaces of the movable and immovable molds while the injection mold is opening; thus, a rush of the high-pressure gas jetted out removes a non-volatile component in the cavity. In addition, such a rush of the high-pressure gas jetted out serves to clean the surfaces inside the cavity and the joint surfaces of the movable and immovable molds.
Preferably, the movable mold may be configured to move at a very low velocity for only an initial period of time while the injection mold is opening, keeping the clearance very small to improve a cavity cleaning effect of the high-pressure gas. Alternatively, the clearance formed between joint surfaces of the movable and immovable molds is restricted to a very small level for a predetermined period immediately after the injection mold starts opening, and the movable mold is allowed to move at a normal velocity after an expiration of the predetermined period. The cavity cleaning effect of the high-pressure gas may be improved in this configuration as well.
The above-described constructions may prevent the joint surfaces of the movable and immovable molds from opening too quickly, so that a high-pressure gas may act on the small clearance formed between the immovable and movable molds for a longer time period. Consequently, the effect of cleaning a nonvolatile component is noticeably improved, and cleaning intervals may be prolonged.
The clearance formed between joint surfaces of the movable and immovable molds may preferably be determined according to viscosity of a material to be formed.
Optionally, an air pressure circuit (not shown) may preferably be provided in the movable and immovable molds to discharge atmosphere in the cavity, and the atmosphere in the cavity may be evacuated through a clearance formed between the joint surfaces of the land and the movable mold, before supplying the high-pressure gas into the cavity; thereafter, a nonvolatile component is discharged using a high-pressure gas as described above. Accordingly, reliability of the cleaning effect may be greatly increased, and a maintenance-free period may be extended longer.
Moreover, there is provided, as another aspect of the present invention, a method of placing an insert in an injection mold, in which a plurality of attraction grippers are used to hold, move and place the insert at a predetermined position in the injection mold.
Provision of the plurality of attraction grippers that hold an insert to place the insert in the projection mold may facilitate precise placement of the insert at a predetermined position in the projection mold because even if one of the attraction grippers fails to hold the insert, the others can hold the insert.
In this construction, the plurality of attraction grippers may preferably be attached to a head of a handling device, so that the head may be operated to place the insert at the predetermined position in the injection mold. Further, each of the plurality of attraction grippers may preferably be configured to hold and release the insert by switching between an attractive negative pressure and a positive pressure. In case where the insert is made of metal materials, the plurality of attraction grippers may preferably be made of electromagnets.
Moreover, as yet another aspect of the present invention, there is provided a method of placing an insert in an injection mold, in which a magnet is embedded at a predetermined position in the injection mold to attract and hold the insert at the predetermined position on an inner surface of a cavity of the injection mold; and in which magnetic shielding is provided between the injection mold and the magnet to restrict a leakage of a magnetic flux into the injection mold.
In this construction, a leakage of a magnetic flux into the injection mold typically made of metal materials may be restricted by the shielding provided between the injection mold and the magnet, and thus a magnetic attraction may be prevented from decreasing. Consequently, the insert may be securely held at the predetermined insert position.
Other objects and further features of the present invention will become readily apparent from the following description of preferred embodiments with reference to accompanying drawings.